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Caledonian and Variscan structures in the Rocroi-Ardenne Lower Palaeozoic basement (Belgium and adjacent countries)
This paper presents a synthesis of the relevant structural evidence for a Caledonian tectonic event in the Ardenne, with special reference to the Rocroi Massif. The importance of the Variscan imprint in the basement is also discussed.
The Lower Palaeozoic basement of Belgium and adjacent countries outcrops in the Brabant Massif, the Condroz Ridge and in the High-Ardenne Massifs. In the Ardenne, the basement was submitted to two successive compressive tectonic events (Caledonian and North-Variscan), separated by a period of tectonic relaxation or distension. Recent microstructural evidence, mainly from the Rocroi and Stavelot-Venn Massifs allows some insight into evolution of these tectonic events.
The Caledonian compressive period is subdivided into by three deformation phases, successively characterized by the formation of a penetrative cleavage, associated with minor N-S folds and extension fractures (D1), by major folding along a regional E-W to ENE-WSW axis (D2) and by late upright folding of lesser importance (D3). The relationship between the first two types of structure has still to be clarified. However, observations indicate that the original cleavage formation might be older than the major E-W to ENE-WSW folding.
The period of tectonic relaxation between the Caledonian and Variscan compression is marked by magmatic intrusions, in extensional setting (D4).
The Variscan deformation in the basement occurred mainly by reactivation of pre-existing Caledonian anisotropies and by the development of new structures of minor importance, like boudinage, folds and mullions (D5). The existing anisotropy was determined by cleavage as well as bedding planes, which were probably already subparallel to each other, as a consequence of the intensity and the isoclinal style of the Caledonian folding. Late and minor effects of the Variscan event (D6) are expressed mainly by kink bands in the Southern zone of the Rocroi Massif, and by open, upright, hectometric-scale, E-W folds in the Northern zone.
The existence of Caledonian deformation, as witnessed by cleavage development and isoclinal folding, was also demonstrated by several authors in the other High Ardenne's Caledonian Massifs.
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A propos de : Damien Delvaux de Fenffe
Laboratoires de Géologie Générale, 3, pl. L. Pasteur, B-1348 Louvain-la-Neuve, Belgium ; Present address: Royal Museum for Central Africa, dept. Geology-Mineralogy, B-3080 Tervuren, Belgium.
A propos de : Dominique Laduron
Laboratoires de Géologie Générale, 3, pl. L. Pasteur, B-1348 Louvain-la-Neuve, Belgium.